In order to obtain fluid absorbent articles which exhibit good wicking ability, a high total and local fluid uptake capacity, good fluid retaining capacity and a high degree of surface dryness, such articles are usually built up of a plurality of different fibrous structures having different functions. The fibrous structures are often produced of inherently hydrophobic materials, e.g. nonwoven materials, and thus such inherently hydrophobic fibrous structures need to be modified to allow fluid uptake. One major problem when constructing fluid absorbent articles of this kind is, however, that it is difficult to obtain optimal wettability, i.e. an optimal degree of hydrophilicity, which is both immediate and durable, i.e. remains unchanged after the article has been exposed to wetting. Furthermore, it is difficult to maintain stable wetting characteristics in absorbent articles, which are stored for an extended period of time.
Regarding fluid permeable cover sheets, which are suitable made of said fibrous structures, for use in absorbent articles such as diapers, incontinence guards and sanitary napkins, wherein the cover sheet is intended to be in contact with the body of a user during use, it is important that the cover sheet immediately exhibits a desired wettability and could stand repeated wettings. In other words, the cover sheet should immediately exhibit an optimal degree of hydrophilicity and remain fluid permeable even after the absorbent article has been exposed to fluid impact several times. Furthermore, it is important that the cover sheet can accept a large amount of fluid during a short interval of time. Another important property of the fluid permeable cover sheet is the ability to exhibit high surface dryness even after having been exposed to several wettings. In order to obtain a cover sheet having the desired properties, it is important that the cover sheet immediately exhibits an optimal, i.e. a desired, degree of hydrophilicity and that the degree of hydrophilicity varies only within a very limited range when the fibrous structure is wetted or when it is subjected to ageing.
As well known to the man skilled in the art, the literature of these fields talk about the properties of “hydrophilicity” or “wettability” of a substrate and often report measurements of “run-off” and “surface tension” to evaluate such properties.
Commonly used methods for increasing the wettability of fluid permeable fibrous structure or of fluid permeable cover sheets both for use as cover sheets in absorbent articles (the same applies to any fibrous structure to be used in absorbent articles) are:
Treatment of the fibrous structure or of the material of said cover sheets with surface active agents, wherein an increased amount of surface active agents has been used to further increase said wettability. Further, treatment with surface active agents here includes use of mixtures of surfactants, e.g. a mixture of a co-wetting agent (reducing surface tension of the fluid which leads to a rapid inlet) and a surfactant having a more durable character, or addition of surface active agents as internal additives.
Exposure of the material of said fibrous structure, or of the material of said cover sheets, to a corona treatment in combination with any addition of hydrophilic substances, methods for corona treatment are for example described in U.S. Pat. No. 5,576,076, U.S. Pat. No. 5,527,629 and U.S. Pat. No. 5,523,124.
The treatment of the fibrous structures or the cover sheets with surface active agents is for example carried out by coating the hydrophobic material with a surface active agent. In order for a material to be fluid permeable the contact angle between the surface of the material and the fluid must be less than 90°. However, a problem in connection with using fibrous structure or cover sheets which have been coated with a surface active agent is that such fibrous structures or cover sheets exhibit decreasing fluid permeability with repeated wetting. The reason for this is that the applied surface active agents are not firmly attached to the surface of the cover material and will be detached from the cover material and solved in body fluid during the first wetting. At subsequent wetting the amount of surface active agent which remains on the surface of the cover sheet is considerably reduced, resulting in impaired fluid permeability. The same applies to absorbent articles when said surface active agents are added as internal additives, because surface active agents added as internal additives do not migrate fast enough to the surface of the absorbent article after wettings of said surface.
Further problems with the treatment of the fibrous structure, or the cover sheets, with surface active agents, besides the lacking durability, are problems which also increases with increased amount of added surface active agent, for example skin irritation which is caused by migration of surface active agents from the cover sheet to the skin of the user, and influence of the absorbent core comprised in said absorbent articles also because of said migration of surface active agents.
Further, commonly used air corona treatment (optionally in combination with addition of hydrophilic substances) does not render any absorbent articles exhibiting an optimal, i.e. a desired, degree of hydrophilicity which is durable. Said degree of hydrophilicity is not durable since said commonly used corona treatment may create a low molecular weight material which is not properly bonded to the surface. Hence subsequent wetting of the surface of the cover sheet may partly remove the created low molecular weight material and added hydrophilic substances and thus resulting in impaired fluid permeability.
In U.S. Pat. No. 5,814,567 a durable hydrophilic coating for a porous hydrophobic substrate is desribed, which may include exposing the substrate to a field of reactive species.
Moreover at a first wetting by a fluid, in U.S. Pat. No. 5,814,567 said hydrophilic coating does not decrease the surface tension of said fluid.
European patent application 98402010.7 (not published) describes fibrous structures which exhibits a well defined rate of wetting and may be useful in absorbent articles. The fibrous structures of the European patent application 98402010.7 have one or more types of polar, silicon-containing compounds bonded to their surfaces but said fibrous structures are not treated with any surface active agent, i.e. no surface active agent at all is added. Thus absorbent articles comprising the fibrous structures described in European patent application 98402010.7 do not show any of the above described problems related to conventional surface active agent treatment or to commonly used corona treatment (optionally in combination with addition of hydrophilic substances).
Nevertheless there is still a need for further modifications of fibrous structures comprised in absorbent articles not showing any of or at least significantly reducing the problems related to conventional surface active agent treatment or to commonly used corona treatment (optionally in combination with addition of hydrophilic substances), especially there is a need for such absorbent articles exhibiting excellent properties of “hydrophilicity” or “wettability” which are both immediate and durable.